1. Field of the Invention
The present invention relates to cooking ovens, and more particularly to a self-cleaning system for an oven which is capable of automatically eliminating food soils accumulated on its walls by a pyrolytic process at a high temperature.
2. Description of the Related Art
It is well known, as disclosed in U.S. Pat. Nos. 3,962,561 and 4,481,404, that cooking ovens such as electric ovens, gas ovens and convection microwave ovens can not only be used for normal cooking but also can pyrolytically eliminate food soils attached to its walls during the normal cooking. The pyrolytic elimination can be effected by heating and maintaining the cooking chamber at a high cleaning temperature--such as between 800.degree. F. -850.degree. F. --for one to four hours. As soils are pyrolytically degraded, exhaust products are produced such as water vapor, carbon monoxide, carbon dioxide and others including partially combusted or degraded soil particles which produce smoke and odors. It is known that a catalyst, located in the exhaust duct of an oven, can be used to further degrade or oxidize the partially combusted products, thereby minimizing smoke and odors that are exhausted into the ambient atmosphere--typically a kitchen.
The typical self clean cycle is effected by having a user select a self clean option. Initiation of the self clean cycle will set a high control temperature for the oven, cause the oven door to be locked immediately or at some predetermined time or temperature and proceed to heat the oven cavity to a relatively high temperature for a predetermined time before ending the heating cycle, allowing cooling to occur and then releasing the door lock as an end to the self clean cycle.
The cleaning time actually needed to pyrolytically clean a cooking chamber greatly depends on the amount of food soils in the cooking chamber. In the case of light food soils, the soil-elimination is sufficiently effected with a relatively short cleaning period, such as a cleaning time of about one hour (about 1/2 hour for heating-up and about 1/2 hour for cooling-off). On the other hand, in the case of heavy food soils, the chamber temperature must be maintained at the cleaning temperature for about three hours. As a result, the cleaning time is about four hours for a worst case/heavy food soil condition (about 1/2 hours for heating-up, about three hours for keeping the cleaning temperature and about 1/2 hours for cooling-off).
In order to ensure that the self clean cycle adequately cleans the oven chamber, the self clean time period is commonly determined by assuming a worst case cycle. Accordingly, the predetermined self cleaning cycle time is generally set at about 4 hours. Setting the self clean cycle under a worst case analysis wastefully consumes energy and unnecessarily requires long cleaning periods.
To address this problem of wasteful energy use during self clean, there have been some past attempts to monitor the progress of a self clean cycle and to control the duration of a self clean cycle to only the time actually needed to degrade and remove soils from the oven chamber. For example, U.S. Pat. No. 4,954,694, discloses a system wherein a gas sensor monitors the exhaust gases produced during a self clean cycle and determines the self clean heating time in response to the exhaust gas component variation.